The present application is related to U.S. patent application ENTITLED xe2x80x9cABSOLUTE POSITION MOIRÉ TYPE ENCODER FOR USE IN A CONTROL SYSTEMxe2x80x9d, filed Apr. 24, 2001 by Michel G. Laberge, Thomas W. Steiner and Valentin Karasyuk.
The invention disclosed herein relates generally to the field of optical switching. Specifically, the invention relates to alignment control systems for implementing the optical signal connection between fibers in optical cross-connect switches.
Fiber optic systems are now in common use for transmitting optical communication signals, which are optical signals modulated to encode desired information. Such optical communication signals may be modulated with data, voice or video signals, and are typically transmitted across optical networks using optical fibers that support substantial transmission capacity. Given the continually escalating demand for improved signal quality and bandwidth, it is anticipated that the use of, and demand for, fiber optic communication will continue to increase.
One of the reasons that fiber optic networks have recently attracted attention relates to new methods of switching in the optical domain without having to convert the optical communication signals into conventional electronic signals for switching purposes. In this manner, conventional electronic switching components can be eliminated and higher bandwidth optical switches can be implemented in their place. Fiber optic communication lines may be switched by simply aligning the opposing ends of the fibers to be connected for direct optical linkage. Optical switching is advantageous when compared to conversion of optical communication signals to electrical signals for electrical switching, because optical switches can achieve higher bandwidth than comparable electronic switches. Consequently, network application having minimum bandwidth requirements can be achieved at overall lower costs by employing optical rather than electronic switching. It will be appreciated, however, that increasing switching speed and reducing signal degradation across the switch remains a continual desire.
Optical cross-connect switches may be understood to include a first array of fibers on one xe2x80x9csidexe2x80x9d of the switch and a second array of fibers on the second xe2x80x9csidexe2x80x9d of the switch. It will be appreciated after reading the disclosure below, that the first and second xe2x80x9csidesxe2x80x9d of a switch relate to optical signal transmission pathways and not to a spatial arrangement.
The control of optical switches involves alignment of the two fibers to be optically connected. Typically, external network information is received, identifying two optical fibers (one from either xe2x80x9csidexe2x80x9d of the switch) that are to be optically connected across a switch interface in order to permit transmission of communication signals there-between. Alignment control involves identifying the first side fiber and the fiber on the second side that are to be optically interconnected, aligning and configuring an optical connection between the two fibers and fine tuning the optical connection between the identified fibers to optimise signal transmission.
Some older prior art optical cross-connect switches employ xe2x80x9cone-sidedxe2x80x9d control systems for targeting and alignment. That is, the control system for aligning a particular transmitting fiber to a particular receiving fiber is resident on one xe2x80x9csidexe2x80x9d of the switch. Such alignment control systems attempt to control the trajectory of the communication signal based on known device geometry and pre-calibrated target positions. One example of such a system is U.S. Pat. No. 5,206,497 (referred to herein as the ""497 patent), which controls the alignment of the transmitting fiber on the transmission xe2x80x9csidexe2x80x9d of the switch. The ""497 patent discloses a fiber that transmits an optical communication signal to one of a plurality of receiving fibers on the other side of the switch. A small fraction of the power in the communication signal is split off from the main component of the beam using a beam splitting device. This split-off component of the beam, referred to as the monitor component, is directed towards a charge coupled device (CCD) optical detector. The CCD detector determines the position of the monitor component of the beam on the surface of the CCD detector and feeds this information back to the alignment control system. Using feedback information from the CCD detector, the alignment control system operates actuators, which effect the desired trajectory of the transmitting fiber. During calibration which takes place prior to use, a predetermined target position is identified, so that each receiving fiber is associated with a known target position on the CCD detector. In this manner, the alignment control system achieves the desired communication signal trajectory by directing the monitor component of the communication signal to impinge on the surface of the CCD detector at the desired target position.
A major drawback with the control system of the ""497 patent is the requirement of a CCD sensor for each transmission fiber. CCD sensors are relatively expensive and increase the cost of the overall switch. An additional disadvantage of the ""497 invention is the space required to split each communication signal to an independent CCD sensor, which prohibits miniaturization of the switch. Another limitation of the ""497 patent is the dependence of the control system on the presence of the communication signal. When the system is xe2x80x9cunlitxe2x80x9d (i.e. no optical communication signal is being transmitted from one side of the switch to the other) it can not be controlled. The requirement for the presence of a communication signal prior to effecting control becomes a problem if it is desired to insert the switch in a communications network prior to use and then use the switch only when additional network capacity is required. The necessity of having the communication signal on during control is also problematic, because a signal may be inadvertently inserted into the wrong receiving fiber during a switching operation. Additional drawbacks with the ""497 patent include: a limitation on insertion efficiency across the switch interface, because of a need to split off some of the transmitted power to form the monitor component of the beam for the control system, and a critical dependence on the geometry of the device.
Another drawback, common to all xe2x80x9cone-sided controlxe2x80x9d switches (like that of the ""497 patent) is that the numerical aperture (NA) of the receiving fibers prevents the implementation of very large switches. Recently, larger switches have been developed by having controllable fibers on both xe2x80x9csidesxe2x80x9d of the switch. That is, control of the communication signal trajectory is implemented on the transmission side of the switch and control of the angle at which the signal is inserted into the receiving fiber is controlled at the reception side of the switch. In this manner, the limited NA of the receiving fibers can be overcome.
An example of a xe2x80x9ctwo-sided controlxe2x80x9d switch is U.S. Pat. No. 6,005,998 (referred to herein as ""998), which comprises two arrays of light beam collimators arranged on either side of the switch. The control system comprises two motors, which each have an associated encoder to track their positions. The motors are used to effect a particular angle of a collimating lens. On the transmission side of the switch, the angle of the collimating lens is adjusted such that the beam of the transmission fiber is collimated and steered on both the x and y axes to a pre-calibrated target position on the other side of the switch. On the receiving side of the switch, two additional motors control the angle of a similar collimator lens. The angle of the collimator lens permits reception of the communication signal from the pre-calibrated target position and insertion of the communication signal into the receiving fiber at an appropriate angle, helping to overcome the limited NA of the receiving fibers.
A major drawback of the ""998 patent is the requirement for two motors in each collimator unit (i.e. two motors for each transmission fiber and two motors for each receiving fiber). These motors are relatively expensive and they take up an inordinate amount of space, preventing miniaturization of the device. Another limitation of the ""998 patent is that the encoders are only capable of determining relative position of the collimating lens; consequently, each time that the system is powered off and then restarted, it must be xe2x80x9czeroedxe2x80x9d to achieve a reference position. Finally, as with the ""497 patent, the system of the ""998 patent is heavily dependent on the relative geometry of the components and if, after calibration, any device is dislocated or if the system geometry changes due to environmental conditions, such as vibration or temperature, the system will fail.
The current generation of optical cross-connect switches employs a xe2x80x9ctwo-sidedxe2x80x9d control scheme optical control signals that are distinct from the communication signals that carry the data to be switched. The switch disclosed in U.S. Pat. No. 5,524,153 (hereinafter ""153) has a target identification and alignment system involving the use of optical control signals associated with the first and second side fibers, where each fiber is housed in a switching unit. LED""s or other radiation emitting devices (referred to herein as RED""s) are interspersed with the fibers in the first and second arrays, thereby defining corresponding arrays of RED""s positioned in a known manner relative to the fibers. Identifying a target fiber within the array of switching units can then be accomplished by lighting the RED""s of the various rows and columns of the RED array in a particular pattern to identify the fiber that is to be targeted for connection. Pulsing of the RED""s also provides a positional reference for fine tuning the alignment of the fibers on each side of the switch. As explained below, such targeting and alignment methods have certain limitations.
One principal limitation of the ""153 patent relates to targeting in large capacity switches, such as a 256xc3x97256 or larger switch. In such instances, a sequence involving a relatively large number of RED pulses is required to identify a particular target fiber. With such a large number of pulses, the operational speed of the switch may be significantly reduced.
A second disadvantage of the invention disclosed by the ""153 patent is the dependence of the alignment control system on the absolute radiation intensity of the measured pulses. The control system taught by the ""153 invention involves a comparison of the absolute intensity of control signal radiation pulses on either side of the target fiber to develop an error signal. This dependence on absolute intensity leaves the alignment control system susceptible to variation in the intensity amongst the various control signal RED""s and variation in the reception of the radiation from the various control signal RED""s. For this reason, one RED that performs differently than the others can influence the successful operation of the device and be a singular point, where a failure may cripple the operation of the entire switch. It may also be difficult to identify the particular switching unit to which the defect belongs, making it difficult to repair a switch with an unusually performing RED.
A third drawback with the ""153 invention is its requirement for another fiber dedicated to receiving optical control signals, referred to in the ""153 patent as a xe2x80x9cradiation guidexe2x80x9d. Each switching unit must contain both a data signal fiber to receive the communication signals and a radiation guide to receive the control signals. The invention discloses two types of radiation guides: (1) a dedicated second fiber that is completely distinct from the data signal fiber; and (2) a dual core fiber, where the data signals are received by the inner core and the control signals are received by the outer multimode core. The implementation involving a second distinct fiber has the disadvantage that both the data signal fiber and the control signal fiber must have access to the aperture of the switching unit. This layout takes up additional space in the lateral dimensions, increasing the overall size of the switching unit and limiting the range of alignment of the data signal fiber. The implementation involving the dual core fibers is relatively expensive, because of the cost of the dual core fiber and the special cable tapping techniques that are necessary to extract the control signals from the outer core of the dual core fiber.
Another limitation of the ""153 patent is the requirement that the alignment control RED""s are associated with, and located adjacent to, the apertures of each switching unit. Locating the RED""s adjacent to the apertures of the switching unit increases the space required by each switching unit, thereby increasing the size of the overall switch, increasing the switching time and limiting miniaturization. In addition, the ""153 invention discloses a group of control RED""s that are associated with each switching unit. It is a requirement for the operation of the switch that each of these control RED""s must be received and measured by the switching units on the opposite xe2x80x9csidexe2x80x9d of the switch. It will be appreciated that in larger switches (such as 256xc3x97256 or greater), the control RED""s associated with the outermost switching units will be a relatively large lateral distance from the center of the switch interface. In order for radiation from these outermost control RED""s to be received and measured by the switching units on the opposite side of the switch, the switching units will require a large NA. Thus, the association of the control RED""s with the switching units limits the size of the switch that can be effected by the ""153 invention for switching units having a particular NA.
U.S. Pat. Nos. 6,097,858, 6,097,860 and 6,101,299 (referred to herein as the xe2x80x9cLaor patentsxe2x80x9d) represent a series of patents related to the same prior art switch. As with the ""153 patent, the Laor patents involve the use of a plurality of optical control signals associated with the first and second side fibers, where each fiber is housed in a switching unit. The Laor patents describe the use of two types of control signals referred to therein as targeting and alignment signals. In the Laor patents, there are a plurality of control signal RED""s associated with each switching unit that emit control signals having a different wavelength from the communication signal, but that travel on a substantially similar optical path to that of the communication signal. Using a dichroic beam splitter, the control signals are split from the communication signal inside the receiving switching unit on the basis of this wavelength difference, so that the communication signal is directed towards the receiving fiber and the control signals are directed toward a sensor apparatus for control purposes. The Laor patents disclose the use of a variety of sensors, involving a single detector, a partitioned detector and a plurality of detectors to receive alignment and targeting signals and to feedback this information to a control system. The Laor patents improve on the ""153 patent by including targeting information (i.e. information identifying the two fibers to be optically connected) in a coded signal from one of the control signal RED""s. Once a targeting signal is decoded, the switching units associated with the two fibers to be optically connected can be actuated in an open loop manner to achieve approximate optical alignment based on a pre-calibrated target position. Thereafter, the control systems can measure the signals received from the alignment RED""s and employ linear feedback control based on knowledge of the system geometry to fine tune the connection for communication signal transmission. The encoding of the alignment signals improves the switching speed of the device, because it does not require a drawn out pulsing sequence of the RED""s in various rows and columns.
Although the Laor patents reduce the switching time by encoding the targeting signal, they suffer from a number of other disadvantages, some of which are similar to those of the ""153 patent. A primary disadvantage of the Laor patents that is similar to that of the ""153 patent is their dependence on the absolute radiation intensity of the radiation pulses from the alignment RED""s. The control system taught by the Laor patents employs a comparison of the measured absolute intensity of various alignment RED pulses on various detectors to determine an error signal used to control its actuators. This dependence on absolute intensity leaves the entire system susceptible to variation in intensity amongst the various control signal RED""s and variation in reception and measurement of the radiation from the various control signal RED""s. For this reason, one RED that performs differently from the others can influence the successful operation of the device.
A second disadvantage of the Laor patents is the relatively large number of parts and optical interfaces within the switching units, both of which are greater than that of the ""153 patent. The large number of parts increases the size, complexity and expense of the overall switch. In addition, the large number of optical interfaces required by the Laor patents decreases the optical transmission efficiency of the switch, because there are losses of optical intensity associated with each interface.
Another disadvantage of the Laor patents is the use of a limited number of alignment RED""s coupled with relatively large radiation detector surfaces. This combination of factors results in a relatively weak control signal strength and a critical dependence on the signal to noise ratio of the detectors. Since only one alignment RED is incident on a detector surface at any given time, the detector must be sufficiently sensitive to detect the radiation from the RED and to produce a corresponding electrical signal that is discernible. Such sensitive detectors are susceptible to background radiation and other sources of noise, particularly when their surfaces are relatively large. Thus attempts to make the radiation detectors more sensitive result in lower signal to noise ratio. The problem of relatively weak signal strength from the single alignment RED and a critical dependence on signal to noise ratio of the detector is exacerbated by any losses that might occur at the beam splitter or any of the other optical interfaces within the switching unit. The Laor patents attempt to address this issue by breaking the sensor into smaller partitions, to improve the signal to noise ratio, but this causes a corresponding increase in the complexity and cost of the radiation detection devices.
In addition to the increased complexity and cost of the radiation detection devices disclosed by the Laor patents, the partitioned sensors of the Laor patents suffer from xe2x80x9cmigrating photonxe2x80x9d effects near the edges of the sensor partitions. Eliminating migrating photons from such detectors represents an additional source of expense required to effect the switches of the Laor patents.
In a manner similar to that of the ""153 patent, the Laor patents disclose control RED""s that are associated with, and located adjacent to, the apertures of each switching unit. As discussed above in reference to the ""153 patent, this design increases the space required by each switching unit, thereby increasing the size of the overall switch, increasing the switching time and limiting miniaturization. As also mentioned above, the requirement that each control RED be received and measured by the switching units on the opposite xe2x80x9csidexe2x80x9d of the switch limits the overall switching capacity of the switch. This limitation arises because each switching unit has a given NA and, in large capacity switches, the radiation from the control RED""s associated with the outermost switching units is difficult for the opposing side switching units to receive and measure.
Another disadvantage of the Laor patents is the potential for stray radiation from the alignment RED""s within a particular switching unit to influence the intensity measurement of the desired alignment signals coming from the opposing switching unit. Because the geometry of the switching units is relatively small and because the alignment RED""s are located inside the switching units, there is a significant potential that radiation emitted by an alignment RED within a particular switching unit can reach the radiation detectors within that same switching unit. As discussed above, the radiation detectors associated with the Laor patents are required to be sensitive to measure the radiation emitted from a single alignment RED on the other side of the switch. This feature of the radiation detectors increases the possibility of measuring stray radiation emitted by the alignment RED""s within the same switching unit. The Laor patents do not appear to disclose any techniques of minimizing the effect of this stray radiation on their control systems.
Current actuation techniques for optical cross-connect switches employ motors (such as the ""998 patent), piezoelectric actuation schemes (such as the ""153 patent and the Laor patents) and specialized electrostatic enabled micro-machine actuators. Each of these actuator implementations suffer from their own unique drawbacks. More specifically, motors take up a relatively large amount of space, a relatively large amount of power, and suffer from backlash. Piezoelectric actuators suffer from hysteresis in their actuation profile and require large voltages, which may not be desirable in a fiber optic switch. Finally, electrostatic micro-machine actuators must be specifically designed for each application; consequently, they are relatively expensive to produce.
It is an object of the present invention to provide a method and apparatus for an optical cross-connect switch having an improved alignment control system, which comprises an improved actuation system and an improved fiber position measurement system.
It is an object of the present invention to provide a method and apparatus for an optical cross-connect switch and an associated alignment control system to minimize the loss of optical power across the switch.
Another object of the present invention is to provide a method and apparatus for an optical cross-connect switching system and an associated alignment control system to maximize the switching speed of the switch.
Yet another object of the present invention is to provide an optical cross connect switch that embodies relatively simple and inexpensive components, so as to minimize the expense associated with the device. In particular, the present invention may be implemented using a single photodetector, rather than a CCD detector or a quadrature detector.
Still another object of the present invention is to provide a method and apparatus for an optical cross-connect switch that is sufficiently robust and well designed to withstand variation in environmental conditions and its component parts. In particular, it is an object of the present invention to overcome vibrations, heat induced changes in the size and orientation of the components, and variation in the intensity of various control signal radiation sources.
Another object of the present invention is to provide a modular optical cross-connect switch that minimizes the lateral space occupied by each switching unit, so as to further facilitate miniaturization.
Still another object of the present invention is to provide a method and apparatus for employing an absolute position encoding scheme in the alignment control system of an optical cross-connect switch. The absolute position encoder is operative to measure the position of a beam directing element, whose position influences an optical path of the optical signal to be switched.
Another object of the present invention is to provide an alignment control system wherein the alignment control system employs control signal radiation sources to establish the position of a beam directing element, but the control signal radiation sources are shared between all of the switching units, rather than the radiation sources being integral to a particular switching unit.
Yet another object of the present invention is to provide a method and apparatus using projected radiation and a reticle to generate a Moirxc3xa9 interference pattern, which pattern yields information about the position of a beam directing element in the switch.
There exists a need for an optical cross-connect switch that ameliorates at least some of the disadvantages of the prior art discussed above.
In another aspect of the present invention, the apparatus comprises first and second groups of optically opposed switching units, which are held in a support chassis. In addition, the switch has a plurality of rigidly mounted radiation sources that emit control signal radiation incident on the face of the support chassis and the various switching units. Each switching unit comprises a housing framework, which houses the fiber and secures it over a portion of its length, but allows an end portion of the fiber to bend. The switch employs an actuation system that receives actuation signals as input and, in response to those actuation signals, bends the end portion of the fiber. A position measurement system associated with each switching unit determines the actual position of the fiber end on two dimensions and a control system uses this information, along with well known control techniques, to calculate actuation signals required to move the end of the fiber to a desired position. The desired position of the fiber end achieves the necessary fiber alignment to effect a switching operation and to facilitate the transmission of optical communication signals across the switch. The position measurement system used to determine the actual position of the fiber end involves a reticle, which is affixed to the end portion of the fiber and has a predetermined spatial relationship to the fiber end. An optical system is used to project the images of at least two of the radiation sources onto the surface of the reticle and a photodetector measures the intensity of the control signal radiation that is transmitted through the reticle. In this manner, the control system is able to resolve a Moirxc3xa9 interference pattern from the portion of the control signal radiation that is transmitted through the reticle and use information contained in that Moirxc3xa9 pattern to calculate the actual position of the fiber end. Once the control system has determined the actuation signals required, the actuation system is operative to bend the end portion of the fiber in a manner such that the fiber end reaches the desired position. When one fiber end from each of the two optically opposed groups of switching units is aligned to its desired position, optical communication signals may be transmitted between the two fibers with a minimum amount of loss or signal degradation.
Preferably, the reticle used in the position measurement system may be positioned and affixed circumferentially around the end portion of the optical fiber. Advantageously, the control system may employ information from the present and historic actual position of the fiber end along with the desired position of the fiber end to determine the actuation signals. Preferably, the chassis may be further divided into two chassis elements, a first of which may hold the first group of optical fiber switching units and a second of which may hold the second group of switching units.
Advantageously, the control system disclosed by the present invention may be further operative to determine an absolute actual position of the fiber end. Such an absolute position measurement system may be effected by a reticle having a constant pitch and a variable aperture duty cycle on two orthogonal dimensions, a reticle having concentric circular non-transmissive areas, or a reticle having a constant pitch and having a periodic variation of aperture duty cycle on two orthogonal directions.
Advantageously, the surface of the reticle may further comprise a plurality of materials, each of which has wavelength dependent transmission properties. In such a case, the plurality of radiation sources may further comprise groups of radiation sources that have different wavelengths.
Advantageously, the surface of the reticle may further comprise a different plurality of materials, each of which has polarization dependent transmission properties. In such a case, the plurality of radiation sources or the optical system may be further operative to introduce multiple polarizations to the control signal radiation.
Advantageously, the control system disclosed by the present invention may be further operative to discern rotation of the end portion of the fiber.
Another aspect of the present invention discloses a method of aligning an optical fiber to a desired position on two dimensions. The method involves the initial steps of: (i) affixing a reticle in a predetermined relation to the end of the fiber in a manner such that the reticle is moveable with the fiber end; and (ii) providing a plurality of radiation sources that emit control signal radiation. The method then requires projecting the control signal radiation onto the reticle in a manner that produces images of the radiation sources on the surface of the reticle and detecting and measuring the resultant Moirxc3xa9 interference pattern of the radiation transmitted through the reticle. The Moirxc3xa9 interference pattern provides positional information of the fiber end on two dimensions, which is used to generate actuation signals. The final step involves moving the fiber in response to the actuation signals. This technique is repeated to ensure that the fiber end moves to the desired position and remains there.
Preferably, the projecting step may further comprise time division multiplexing pulses of control signal radiation from various groups of radiation sources within the plurality of radiation sources and the detecting step may further comprise de-multiplexing the pulses from the various groups of radiation sources. This technique may involve the additional step of providing timing information about the multiplexing and de-multiplexing of the pulses of the various groups of radiation sources.
Another aspect of the present invention involves a method and apparatus for a position measurement system for determining a position on two dimensions of a moveable end portion of a fiber in an optical cross-connect switch. The position measurement system comprises a reticle affixed to an end portion of the fiber in such a manner that it has a predetermined spatial relationship to the fiber and is moveable therewith. A plurality of stationary radiation sources emit control signal radiation and an optical system projects the control signal radiation onto the surface of the reticle, so as to form images of the radiation sources on the surface of the reticle. A photodetector measures the intensity of the control signal radiation transmitted through the reticle and a control system discerns a Moirxc3xa9 interference pattern from this measured radiation. Using information contained in the Moirxc3xa9 interference pattern, the control system determines the position of the fiber on two dimensions.
Still another aspect of the present invention is an actuation system for moving the end of a fiber in an optical cross-connect switch on two dimensions. The actuation system comprises a housing framework, which houses the fiber and secures it over a portion of its length, but allows an end portion of the fiber to bend. A magnetically responsive element is affixed to the end portion of the fiber and is moveable along with the end portion of the fiber. An actuation element is disposed around a circumferential perimeter of the fiber in a location near to the secure part of the fiber. The actuation element further comprises a plurality of actuator branches, which are disposed at regular intervals around the circumferential perimeter of the fiber and extend toward the end of the fiber. Each actuator branch has a coil of wire wrapped around a portion of its length. Finally, a plurality of current sources is operative to deliver current to the coils of wire wrapped around the respective actuator branches. Delivery of current to the coils of wire creates a magnetic field in the vicinity of the actuator branch, creates a magnetic force on the magnetically responsive element, and moves the fiber end.
Advantageously, the actuation element may be made out of a magnetically polarizable material.
Preferably, the actuator branches may extend past the end of the fiber, so as to create a component of force oriented parallel to the central axis of the secure part of the fiber. This force component prevents the fiber from bending in more than one location.
Still another aspect of the present invention involves a method of moving the end of a fiber in an optical cross-connect switch in two dimensions. The method comprises the steps of: (i) providing an actuation element, which is disposed around a circumferential perimeter of the fiber in a location near to the secure part of the fiber. The actuation element further comprises a plurality of actuator branches, which are disposed at regular intervals around the circumferential perimeter of the fiber and extend toward the end of the fiber. Each actuator branch has a coil of wire wrapped around a portion of its length; (ii) providing a magnetically responsive element, which is affixed to the end portion of the fiber and is moveable along with the end portion of the fiber; and (iii) introducing current into the coil of wire around at least one of the actuator branches, so as to create a magnetic field in the vicinity of the actuator branch, create a magnetic force on the magnetically responsive element, and move the fiber end.
These and other objects of the present invention will be better understood from the following more detailed description along with the drawings and the accompanying claims.